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Discussion Starter #1
I just replaced the gas tank in my 73 and the gauge is stuck on full. Before I changed the tank the gauge would only work till it hit 1/4 tank then it would stop. Any suggestions. Thanks!
 

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correct me if I'm wrong but I would check to make sure the connections are correct...I think the guage shows full when it sees 90 ohms or more of resistence...if either of your wires were disconnedted somewhere in the circuit or the ground itself was bad it could see way more then 90 ohms all the time so the guage would never move....I hope I explained that corectly:eek:
 

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Is there any way to troubleshoot that? When I installed the new tank I plugged in the wire that connects to the top of the sending unit, cleaned up the ground connection and attached that. What wire would cause the gauge to read full?
 

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are you using the same sending unit? I swore I read somewhere if you change out the sending unit you should have the guage recalibrated, what ever that means, I could be wrong tho.
 

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ddemaio said:
Is there any way to troubleshoot that? When I installed the new tank I plugged in the wire that connects to the top of the sending unit, cleaned up the ground connection and attached that. What wire would cause the gauge to read full?
I believe it would be the ground wire that would cause it to read full all the time...possibly custom Jim will see this thread and pipe in...he normally would have a ton of trouble shooting tips for this kinda thing...
 

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I did use the same sending unit just cleaned it up a bit and changed the mesh filter. The ground connection when I disconnected it was full of crud and undercoating, so I sanded the contact points real good and reconnected it. Other than that nothing was different. The odd thing is that the gauge before I removed the tank would only work till it hit 1/4 tank then it would stop.
 

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If the sending unit wire is disconnected or has very poor continuity (added resistance) the fuel gauge will read full or past full. If the fuel sending unit wire is grounded the gauge will read empty or below.

Was the new sending unit checked for proper operation before it was installed ?. I've been caught this way with problems by not checking things and assuming things are good even when the part is new and they weren't.

What you need to do now for the least amount of work is to find the sending unit wire in the trunk and disconnect it. One side of the connector will pass through a grommet in the floor and go towards the gas tank. The other half of the disconnected connector will go into the taillight wiring and up to the dash. This side of the connector needs to be worked on first.

Turn the ignition to the run position and with a jumper wire connect the one end to a good ground and the other end to the connector that goes forward to the gauge. The gauge should swing to empty or below. Now disconnect this jumper wire and the gauge should fo to full or above. If so the gauge is working. If you have access to resistors, connect one side of a 45 ohm resistor to the sending unit wire towards the gauge and the other side of the resistor to ground. The gauge should then read 1/2 tank. If you swap the resistor for a 22.5 ohm then the gauge should read 1/4 tank and if replaced with a 67.5 it should read 3/4 tank. If a 90 ohm is inserted it should read a full tank.

If the gauge reads right in the above tests then look at the ground wire connection off of the tank to the body of the car. If it looks bad where the wire is crimped onto the connector, cut the connector off, crimp on a new ring terminal end, solder it, and reattach it to the car and see if the gauge reads right. If not then the tank has to be dropped for further troubleshooting.

If the tank is dropped, remove the sender and rig it up under the car to where when the float is hanging fully down the gauge should read empty and is the float arm is pushed upward the gauge should read higher and when the float arm is all the way up the gauge should read full.

Here are some shots of some bad spots on a sender I worked on:

http://hometown.aol.com/krystaldesigns/pageA14.html

Jim
 

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Discussion Starter #8
Thanks for the info Custom Jim. That's what I needed. I'll try what you said and see what happens.
 

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I had the same problem on my 72 where it always read above full.

Follow the tan wire along the driver side sill and up through the trunk. It plugs into a connector right near the driver tail lamp. If the spade is loose or disconnected from the connector you will always read full. I found that mine was loose and resecuring it did the trick. Just something else to look out for.
 

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Custom Jim said:
If the sending unit wire is disconnected or has very poor continuity (added resistance) the fuel gauge will read full or past full. If the fuel sending unit wire is grounded the gauge will read empty or below.

Was the new sending unit checked for proper operation before it was installed ?. I've been caught this way with problems by not checking things and assuming things are good even when the part is new and they weren't.

What you need to do now for the least amount of work is to find the sending unit wire in the trunk and disconnect it. One side of the connector will pass through a grommet in the floor and go towards the gas tank. The other half of the disconnected connector will go into the taillight wiring and up to the dash. This side of the connector needs to be worked on first.

Turn the ignition to the run position and with a jumper wire connect the one end to a good ground and the other end to the connector that goes forward to the gauge. The gauge should swing to empty or below. Now disconnect this jumper wire and the gauge should fo to full or above. If so the gauge is working. If you have access to resistors, connect one side of a 45 ohm resistor to the sending unit wire towards the gauge and the other side of the resistor to ground. The gauge should then read 1/2 tank. If you swap the resistor for a 22.5 ohm then the gauge should read 1/4 tank and if replaced with a 67.5 it should read 3/4 tank. If a 90 ohm is inserted it should read a full tank.

If the gauge reads right in the above tests then look at the ground wire connection off of the tank to the body of the car. If it looks bad where the wire is crimped onto the connector, cut the connector off, crimp on a new ring terminal end, solder it, and reattach it to the car and see if the gauge reads right. If not then the tank has to be dropped for further troubleshooting.

If the tank is dropped, remove the sender and rig it up under the car to where when the float is hanging fully down the gauge should read empty and is the float arm is pushed upward the gauge should read higher and when the float arm is all the way up the gauge should read full.

Here are some shots of some bad spots on a sender I worked on:

http://hometown.aol.com/krystaldesigns/pageA14.html

Jim
Sorry to hijack the thread but I found it on a search. The fuel gauge in our 63 Nova wagon quit on us today. With the key on it falls to empty with the key off it rests at half full. So if I understand this it means the sending wire is grounded? Is this correct? Can I just run another wire from the sending unit to the gauge or not? Thanks for the help. ccnova
 

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ccnova said:
Sorry to hijack the thread but I found it on a search. The fuel gauge in our 63 Nova wagon quit on us today. With the key on it falls to empty with the key off it rests at half full. So if I understand this it means the sending wire is grounded? Is this correct? Can I just run another wire from the sending unit to the gauge or not? Thanks for the help. ccnova
It could be the wire or it could be in the sending unit. Try this, disconnect the terminal on the sending unit. I'm not familiar with the older models but I think the sending unit can be accessed without dropping the tank but may have to be to remove it. Most gauge connections off of the sending unit pushed on and if so it should pull off. Some were bolted onto a threaded stud.
With the wire disconnected at the sending unit the gauge should read above full when the key is in the run position and if it is grounded the gauge should go to empty. Zero resistance/dead short should make the gauge read empty while infinite or 30 ohms on a 0-30 ohm gauge or 90 ohms on a 0-90 ohm or disconnected would make the gauge read full or above.
I'm not sure why some gauges change their reading when the key is off. It might be giving us a hint but I don't know what it is. Mine does on my 77 but it reads correctly when close to empty but seems to stay anywhere above half tank to full longer than it should and then once it gets to half tank it seems like it falls faster (not in a close relationship as to what actually is in the tank).

Jim
 
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